Pressure responsive pilot control valves act to provide a signal when a high or low pressure condition is sensed within the flowline or process to which such pilot valves are operably connected. A high pressure condition may be created in the line by a blockage and in such event, the well head pressure may exceed the line rating which can result in line rupture. A low pressure condition may be created when there exists leakage in the line. In either case, it is imperative that a signal be sent that will terminate the flow within the oil or gas line to prevent spills and damage to equipment. For example, the signal forwarded by the pilot control valve may conveniently be pneumatic or hydraulic and such signal may be passed to a latching relay valve which, in turn, controls a shut-in valve which can close and terminate pipeline flow.
At the present time, there are pilot valves being marketed which are operable to sense only a high or only a low pressure condition but not both conditions. If both high and low pressures are to be sensed with present apparatuses, two pilot valves may be required, one to sense the high condition and the other to sense the low condition. Each pilot requires its concomitant manifold and associated hardware with the result that the equipment is unnecessarily expensive and bulky.
Certain other pilot valves are being marketed which do sense high and low conditions but which are incapable of responding to the range of pressures which can be covered by the pilot valve according to the present invention.
A further problem with present equipment is that, often, the pilot valve must be operated in conditions that are severe such as those areas offshore and in the coastal regions such as the Gulf Coast. Certain construction materials are particularly vulnerable to such operating conditions with the result that they corrode over time and operate unsatisfactorily under such conditions. Also, certain forms of construction of pilot valves are not readily manufactured at reasonable cost with the corrosion resistant materials that are required. Thus, they are too expensive for many applications. In addition, some pilots, in utilising O-rings for sealing that move over the ports, subject the O-rings to flexing, inconsistent loading and wear with the result that failures and breakdowns may more frequently occur.
A further disadvantage in respect of many latching relay valves used in association with pilot valves, including the trip valve shown in our U.S. Pat. No. 4,961,560 is that they are typically used for lower pressures and, in such event, a spool and O-ring may be used. It is difficult to have an O-ring seat on both the body and spool of a relay valve and, further, the seal may lack integrity at higher pressures when a large area of the O-ring is exposed to such pressures. This is disadvantageous when the relay valve is intended to be used under conditions of higher pressure such as when they are used for flowlines transporting gas.